Using fake MX records to combat spam

Question

I have a client that is getting heavily spammed.. it's the 15th of the month and POP3 bandwidth is almost 100 GB (yes GB..lol!) there are only 7 e-mail account on this domain. I installed SpamAssassin set it to 5 and setup 10-20 filters reject most of the junk. I don't see much of a change in POP3 bandwidth. Correct me if I'm wrong, the server still receives the message using up bandwidth in order to analyze determine a spam score.

I stumbled across faking MX records, for thoes unaware--basically you set a bogus server as the lowest and highest MX records with the working server's MX record in the middle.

For example:

fake.mydomain.com    1
realmx.mydomain.com  2
fake2.mydomain.com   3

The theory is, since majority of the spam is generated from Windows-based zombies and quite a few will query for the the highest MX record to spam since usually they're usually backup servers that don't filter spam. The lowest fake MX-record is for the rest of the spammers.. and generally spammers don't retry after failures.

Has anyone tried this? Does it help? Does it delay or cause issues with mail delivery? Does anyone else have a better solution?

Answer 1

Do yourself a favor and set them up with a gateway anti-spam service such as Postini. For a few dollars per mailbox per month, there's absolutely no reason not to and you'll not only eliminate 99% of your spam, you'll also enjoy having access to their spool service (handy for scheduled or unscheduled downtime), not to mention the bandwidth savings by letting someone else receive and process all that spam before it hits the edge of your network.

Not a Postini employee, just a happy user who's also setup dozens of clients with it.

Answer 2

I've tried this, and I can strongly recommend that you DON'T DO IT! It seemed like a good idea at the time, but after mail from various senders starting disappearing, I realized that it was a mistake. What I didn't realize was that there are lots of terribly written SMTP servers out there, that don't follow the spec and are fairly retarded about handling errors, and people don't know or care because "this other guy got my email, so it must be you".

I second some of the other suggestions for handling SPAM. Postini is a great service, and even the built in anti-spam stuff in the free google apps isn't that bad. If you want more control you can buy an IronPort or other device, or roll your own.

Answer 3

I've never heard of this method before and I can imagine it would delay legitimate email potentially by several hours. At the end of the day, the smtp protocols need to deliver your legitimate email. The valid servers will hit the bogus mx record and try to deliver to that server... I don't know what you might have running there (if anything), but they will keep trying until it's accepted.

Proper servers will keep trying the MX records until the mail is delivered. Spammers tend to get smarter and if this works for some spam software now, I doubt it'll work for long. I can't recommend it.

My suggestion is instead to look at using an smtp tarpit in addition to your existing spam filter. There are a number of these available now. I think you'll find it's much more effective than the fake mx record method.

Such tarpits come with smtpd on BSD. There are also some tarpitting features in sendmail 8.13.

Basically, a tarpit works by tying up spam server resources. They do that by delaying the responses they get. e.g. the spam server connects and receives about 1 byte per second.
Some of the tarpit servers look for spam patterns and can recognize a spam server. Legitimate servers will be prepared to wait through a slow response. In some tarpits servers they move the legitimately recognized server onto a whitelist automatically so there is no delay in the future.

Google SMTP Tarpit and take a look.

Answer 4

You didn't mention it, so is there a reason you're not using a DNSBL?

Edit: SpamAssassin includes support for a few of them - without them, you'll be wasting a lot of CPU cycles analyzing spam.

Answer 5

As far as mail filtering goes, I've been vary happy with combination of Spamassasin and policyd-weight, which checks sender hostname and blocklists during SMTP connection. That is a great thing for two reasons:

1. you don't have to process the rejected e-mail with spamassasin, which spares you system resources (bayesian analysis takes some time) and bandwidth
2. sender hosts get rejected, so in the unlikely event of blocking legitimate e-mail its sender gets a delivery failure notification

I'm using the setup on Postfix, but supposedly there is a way to install policyd-weight with Exim.

Answer 6

I didn't get completely the idea, honestly.

Ok, I'm saying my primary mailserver is Fake. Then so? Doesn't it exists at all or what? (Let's suppose it at last cut part of SPAMers either way.) The "survivors" would use secondary — no problem. But why there's 3rd server in this setup?

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Since this supposed to be mine answer, not question, I'd conclude so: it's sick and a pale shadow of Greylisting. If you wanna see real effect try using Greylisting, man.

Answer 7

I drop most of my spam by delaying connections to hosts are listed in the Spamhaus zen list. Spambots don't like delay. Detecting obvious server forgeries in the HELO command also clears out a lot of spam. Conditions I have found to indicate server forgeries include.

• Using my hostname or IP address.
• Using an unqualified hostname.
• Using a domain literal ([192.0.2.15]) instead of a FQDN. (Yes the RFCs require it, but these days it is not used by Internet mail servers.)
• Failing SPF for the HELO name not Mail (I block on fail, softfail, and neutral).

If you value automated or marketing mail, check on the HELO command that don't work include. My experience is that all other mail passes these conditions.

• Using a second level domain name rather than a FQDN for the host.
• Requiring the IP or HELO name to verify rDNS.
• Requiring a valid second level domain for the FQDN. (local is not a valid domain nor is localdomain.)

Signing your return path allows you to block some spam. Although I am seeing far fewer faked bounces recently.

Unfortunately, I find a high percentage of legitimate automated or marketing mail forges their return path. These hosts often don't have a valid postmaster address either. I do find that requiring a valid domain in the return path is workable. I get far more SPF fail response on legitimate email than spam.

I recently posted my experiences with blocking spam with Exim

Answer 8

Besides the lost e-mails from legitimate people with broken gateways, it's been tried a long time ago (like 15 years ago +/-) and the spammers adapted to it almost immediately back then. I suspect it will prove to be a net loss to your e-mail reliability while having little if no impact on spam. However, should you try it, please send us the results!

Answer 9

Unfortunately, there are certain carriers out there that won't send you mail if the first MX record isn't reachable. I recently wrote up my experiences with this on a blog entry so I won't repeat it here. The summary though is that my first MX record was actually an IPv6-only MX record since I figured spammers don't use IPv6 (yet). Unfortunately, this caused problems and in the end I had to add a IPv4 address to the first MX record in my zone.